In an economy that supports the buying and selling of information, vendors need to be able to let their customers know what they have for sale, without giving away too much of the valuable information. Buyers need to be able to determine the characteristics of information they might want to purchase, without having to trust the claims of the sellers. For some purposes, informal or partial methods work: a magazine subscriber is willing to risk that future issues may be poorer in quality than past ones; a database provider may be willing to allow a potential customer a number of free trial searches; and so on. But for other applications, it is highly desirable for a potential buyer to be able to reliably determine various facts about a piece of information before buying it, and absolutely critical to the vendor that the buyer not be given actual read access to it.